Factors Affecting the Distribution and Abundance of Bull Trout: An Investigation at Hierarchical Scales

Abstract

The reported declines of many stocks of bull trout Salvelinus confluentus in the Pacific Northwest has generated much interest in developing conservation and management plans to protect and rebuild populations. These plans require knowledge of the specific requirements of bull trout throughout their range. We describe the relationships between distribution and abundance of bull trout and physical and biotic factors across a large portion of their historical range, We surveyed 1,057 randomly selected sites from 93 streams within 18 major drainages throughout Washington, Idaho, and Montana for the presence of bull trout. We used logistic regression to assess the relationship between the occurrence of bull trout and several physical and biotic factors at site and habitat scales of analysis. Robust regression assessed relationships between densities of bull trout and physical parameters at site, stream, and basin scales of analysis. Bull trout occurred significantly more often in sites within alluviated lowlands and valleys and in sites with undercut banks, large substrates, pools, and where trees and shrubs were the dominant riparian vegetation. Bull trout occurrence at the site scale was inversely related to the percentage of canopy cover and vegetation overhang and the presence of brook trout S. fontinalis and rainbow trout Oncorhynchus mykiss. At the habitat scale, bull trout most often used large, deep pools that lacked extensive canopy cover. They rarely used fast-water habitats with fine sediments, extensive canopy cover, and brook trout. Bull trout densities correlated positively with pool depth, undercut banks, and diverse gradients, and indirectly with fine sediments at both the stream and site scales of analysis. In addition, high densities of bull trout correlated with less vegetation overhang and greater, but variable, percentages of wood and boulder cover at the site scale. The combinations of variables that correlated significantly with bull trout densities varied considerably among different basins. Additionally, the amount of variation in bull trout densities explained by significant variables decreased at finer scales of analysis. These results indicate a hierarchical relationship between the distribution and density of bull trout and physical variables. Thus, land management for bull trout enhancement or protection should be site-specific and tailored within a similar hierarchical framework.